1. Field of the Disclosure
The present disclosure relates generally to wireless charging, and more specifically, to a wireless power receiver and methods for generating a load of the wireless power receiver in a wireless charging system.
2. Description of the Related Art
A mobile terminal, such as a mobile phone or a personal digital assistant (PDA), is generally powered by a rechargeable battery. To recharge the battery, a separate charger is used. Typically, the charger and the battery have separate contacts for electrical connection therebetween.
However, these type of contacts, which normally project outward, may be easily contaminated by foreign bodies, which may inhibit the battery charging.
To address such issues, wireless or contactless recharging techniques have been developed.
Such wireless recharging technology adopts wireless power transmission/reception. For example, wireless recharging allows a mobile phone to be automatically recharged when placed on a recharging pad.
By eliminating the use of a physical connector and/or cable, wireless recharging provides a number of advantages including enhanced waterproofing and portability.
Various examples of wireless charging include using electromagnetic induction, resonance, and radio frequency (RF)/microwave radiation.
Currently, inductive charging is fairly common, but by its nature, has limited applications.
Inductive charging (or magnetic induction) transfers electric power between a primary coil and a second coil. Movement of a magnet near a coil induces an electric current that creates a magnetic field on a side of a sender, and a variation in the magnetic field enables an electric current to be induced on a side of a receiver. Such inductive power transfer presents high energy transfer efficiency.
Resonant charging transfers electricity within a range of a few meters between a charger and a target. Basically, resonated electromagnetic waves carry electrical energy instead of sound. Resonated electric energy directly transfers only when there is an object having the resonant frequency, and an unused portion is absorbed back into the electromagnetic fields, rather than spread in the air. This is beneficial as it does influence ambient machines or human bodies, unlike other electromagnetic waves.
Wireless charging using microwave radiation is capable of long-distance transfer of power and is expected to gain more popularity in the future.
In order for a wireless power transmitter (or a power transmission unit (PTU)) to detect the placement of a wireless power receiver (or a power receive unit (PRU)), the PTU senses a variation in impedance.
When sensing the placement by the PTU, too small a threshold for an impedance variation may result in faulty detection, and too large threshold may lead to detection failure for small objects.
Further, if an impedance difference between the PTU without the PRU placed thereon and the PTU with the PRU place thereon is not substantial, the PTU may not correctly sense a variation in the load. That is, the impedance difference created when the PRU is placed on the PTU should be large enough for the PTU to detect the load variation.
Further, conventional detection methods have difficulty in detection because when the PRU is placed on the PTU, the resistance may vary but the variation in power due to the load variation may be too small to detect. Further, there may be spots on the PTU where the reactance does not change.
For example, when a load variation is caused at an alternating current (AC) terminal of the PRU, because different impedance variations are measured depending on combinations of the PTU and the PRU, if more load-pull power is consumed in the amplification unit of the PTU, the PTU may be subject to negative influences such as overcurrent or overvoltage.
Another conventional method in which a load variation occurs at a direct current (DC) terminal varies only resistance. Thus, if the value is not large, the PTU has difficulty in load detection, and if the value is too large, more standby power consumption may occur because more power should be transferred to a long beacon of the PTU.